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Abstract

This thesis investigates the growth rate and oxygen isotope (δ18O) proxy records of stalagmites. Fluctuations in cave atmosphere carbon dioxide concentration influence the rate and seasonality of calcareous speleothem deposition; spatiotemporal variability in CO2 may therefore have implications for palaeoclimate proxy records derived from stalagmites. Spatial and temporal variability in stalagmite growth rate was modelled, based on a recent high-resolution spatial survey and hourly logging of cave air CO2 and temperature at Scoska Cave (UK). The maximum modelled vertical stalagmite growth rate is 0.22 mm a-1, which varies temporally in response to high-frequency fluctuations in cave air CO2. The relationship between initial dripwater Ca2+ concentration and cave atmosphere CO2 therefore governs the spatio-temporal distribution of calcite deposition versus dissolution in the Scoska system. Modelling further suggests that ventilation may influence the δ18O of precipitating calcite. As a result of CO2, temperature, and growth rate variability, synthetic oxygen isotope records from separated, simultaneously-growing stalagmites differ. One modelled calcite δ18O record correlates well with external temperature; another resembles the same external temperature dataset when both time series are smoothed, indicating that the location of stalagmites within a cave system may be an important factor in the recording of climate signals. Furthermore, the vertical growth of a stalagmite from New St Michael’s Cave (Gibraltar) was modelled based on data from multi-annual cave monitoring and dripwater sampling. Finally, a novel forward model, simulating oxygen isotope fractionation through Earth’s hydrological cycle, quantifies the importance of source oceanic conditions and rainout on stalagmite δ18O. This model replicates instrumental precipitation isotope data and reproduces a modern stalagmite δ18O record with varied success. The model is sensitive to rainout, manipulation of which is alone capable of achieving satisfactory proxy data replications. The model is also integrated with a published mixing model to account for karst hydrological processes.